1. Field of the Invention
The invention relates to a compensation circuit for current control oscillator and particularly to a compensation circuit adopted for use in digital current control oscillator to improve resolution of oscillation frequency and frequency curve and provide stable frequency output for the digital current control oscillator to prevent unlatching phenomenon from occurring in certain frequency zones of the digital current control oscillator.
2. Description of the Prior Art
Refer to FIG. 1 for the basic principle of a conventional technique adopted on digital current control oscillator. In a conventional digital current control oscillator (as shown in FIG. 1), at the initial state the discharge transistor switch 13b in ON and the charge transistor switch 13a is OFF. The discharge power supply 11b provides a ground channel to enable the discharge power of the equivalent load capacitor 15 be greater than the charge power. Hence the potential of the first signal point m1 increases while the potential of the second signal point m2 drops. When the voltage of the second signal point m2 is below the low threshold voltage (VTL) of a controller 100, a feedback signal is generated to turn off the discharge transistor switch 13b of the equivalent load capacitor 15, and to turn on the charge transistor switch 13a at the same time. The charge power supply 11a provides the required power supply. Thus the charge power of the equivalent load capacitor 15 is greater than the discharge power. Therefore the potential drop of the first signal point m1 causes the potential increase of the second signal point m2. When the voltage of the first signal point m1 is greater than the high threshold voltage (VTH) of the controller 100, a feedback signal is generated to turn on the discharge transistor switch 13b of the equivalent load capacitor 15, and to turn off the charge transistor switch 13a at the same time. Thus the discharge power of the equivalent load capacitor 15 is greater than the charge power. The phenomena set forth above repeatedly occur, and an oscillation is generated.
Refer to FIG. 2 for the circuit of a conventional digital current control oscillator. The charge transistor switch 13a consists of a plurality of P transistors connecting in series. The discharge transistor switch 13b consists of a plurality of N transistors connecting in series. The controller 100 controls the number of the switches thereby to control current and oscillation frequency. The controller 100 has a trigger 20 to control the high threshold voltage value and the low threshold voltage value, thereby to control charge and discharge voltage to determine charge and discharge current, and consequently to adjust the oscillation frequency of the oscillator. For instance, when the charge voltage value transmitted to the controller 100 from the equivalent load capacitor 15 reaches the high threshold voltage of the trigger 20, a discharge process occurs, and the trigger 20 provides high low potential signals to the PMOS controller 23a and NMOS controller 23b through an inverter 21. Meanwhile the P bits controller 27a and N bits controller 27b determine the switch number of P transistor 25a in each charge transistor switch 13a and the switch number of N transistor 25b in each discharge transistor switch 13b. The P bits controller 27a connects to PMOS controller 23a through a P controller line p2. Accordingly, a low potential signal transmitted to the charge transistor switch 13a through the P bus p1 is OFF. The N bits controller 27b connects to the MNOS controller 23b through a N control line n2, and a high potential signal is transmitted to the discharge transistor switch 13b through the N bus n1 to make the discharge transistor switch 13b in an ON condition. Based on the aforesaid explanation, when the potential of the first signal point m1 is greater than the second signal point m2, it is a discharge process. The P connection end p3 and N connection end n3 transmit 0 and 1, i.e. OFF and ON signals.
The foregoing setup has problems. For instance, when the charge power is constant, the switch of discharge power is affected and consequently determines variation time of the potential. Same phenomenon occurs vice versa. Hence controlling these switches affects the frequency output of the digital current control oscillator. For example, if only one P transistor 25a is ON in the charge transistor switch 13a, and five are ON the next moment, because of the existence of the switches and different number of the switches at different stages of the process, the switching of the switches will generate a parasite load capacitance. This will affect the potential variation time and result in undesirable frequency output. The invention aims at providing a compensation circuit for current control oscillator to overcome the problem of parasite load.